Abstract

Introduction: Trastuzumab is an iconic example of rationally designed targeted therapy for HER2-positive breast cancers; however, trastuzumab resistance is a imposing clinical challenge that calls for novel approaches to better benefit patients. The purpose of this study was to find alternative combinatorial targeted therapies to overcome trastuzumab resistance.

Methods: We used two distinct PTEN-loss mediated trastuzumab resistant mammary tumor mouse models. Cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and Akt inhibitor triciribine (TCN) were applied alone or in combination with 7.16.4 antibody, the mouse equivalent of the trastuzumab, for the treatment of tumor-bearing mice. Immunoblotting, immunohistochemistry, in vivo cytotoxicity assay and quantitative real time PCR were employed to dissect the in vivo effects of the treatments and molecular changes.

Results: Concomitant targeting of tumor cells with Akt inhibitor TCN plus trastuzumab, and activation of T cells with anti-CTLA-4 antibody in the tumor microenvironment results in a synergistic inhibitory effect on tumor growth and overcomes trastuzumab resistance in both mammary tumor mouse models. In vivo combinatorial treatment with the 7.16.4 HER2/Neu antibody and TCN effectively inhibited tumor growth in both models via inhibiting PI3K/AKT and MAPK signaling accompanied by increased T cell infiltration in the tumor microenvironment. We demonstrated that both CD8+ and CD4+ T cells were essential to the optimal antitumor effect of the combination treatment in an IFN-γ-dependent manner. Importantly, the antitumor activities of HER2/Neu antibody and TCN combination treatment were further improved when we blocked co-inhibitory receptor cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) to enhance the T cell response.